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Abstract

Anthropogenic acid deposition has the potential to accelerate leaching of soil cations, and in turn, deplete nutrients essential to forest vegetation. The critical load concept, employing a simple mass balance (SMB) approach, is often used to model this process. In an evaluation under the U.S. Forest Service Watershed Condition Framework program, soils in all 6th level watersheds on the Huron-Manistee National Forests (HMNF) in Michigan were assigned the lowest score of "3—Impaired Function" due to exceedance of the critical load of acidity as determined by national-scale estimates. The impetus for this research was to test the relevance of national-scale critical acid load estimates at the 6th level watershed scale by using site-specific field data in the SMB model where possible. The Osborn Creek watershed on the HMNF served as a case study. Field data were collected to estimate soil mineral weathering rates, nutrient uptake rates, and forest growth characteristics at five sites containing sandy, nutrient-poor soils. Critical acid loads and exceedances were developed under "best" and "worst" case scenarios given the uncertainty in the SMB model. Despite the high likelihood of actual exceedance and some evidence for soil acidification across the watershed, base saturation remains excessively high (>100 percent) at most sites. Other field data suggest that these soils receive significant external inputs of base cations that may outweigh what is produced through weathering onsite within the rooting zone. Trees show no visible signs of decline. Overall, the SMB approach may not adequately capture the complexity of nutrient cycling at all of the sample sites. The variability of soils, weathering estimates, and nutrient uptake rates between and within sites makes extrapolation of these results to other HMNF watersheds difficult to justify. Management programs aimed at improving our understanding of base cycling in complex glacial terrain, as well as mitigating the risks associated with nutrient depletion from frequent timber harvests and fuels reduction practices, are suggested.